This invention relates to hot rolling and treating rod products. Particular emphasis is placed on plain carbon and alloy steel rod, and a method and apparatus for the treatment thereof after hot rolling to provide a wide choice of physical properties for the rod at far less expense than by conventional rolling and heat treating processes. Also involved is hot rolling steel rod (as well as other rods) followed directly by batch type operations including coating, baking and/or heat treating.
The normal practice in the production of steel rod is to roll it at as high a tonnage rate as possible, to coil it either directly into a bundle (or onto a moving conveyor and thereafter to bundle it), to cool it to room temperature, either before or after bundling, to store it, and eventually to cold work or machine it as may be needed for the production of an end product. Except in the case of concrete reinforcing rod, and a few other minor applications, "as rolled" steel rod is not regarded as an end product in itself. Thus, in a large majority of cases the rod is cold-worked by wire drawing, cold heading and the like processes, and in some cases it is machined as well.
Due to the fact, that rod which is cooled in bundle form after rolling is highly non-uniform in its physical properties, the usual practice, with steels having medium to high carbon content, is to subject the rod to heat treatment by processes such as air or lead patenting, and sometimes by subcritical annealing to render the rod suitable for such cold working. When medium to high carbon content rod is cooled in open ring formation on a moving conveyor as by the process of U.S. Pat. No. 3,321,432, a much more uniform product is obtained which is suitable for cold drawing into various wire and spring steel products without the need for intervening heat treatment. The process of U.S. Pat. No. 3,321,432 has largely eliminated air patenting, but lead patenting is still practiced for certain special end uses. In addition, subcritical batch annealing is also employed in some cases for low carbon rods, and substantial tonnage of low carbon wire is process annealed.
One form of "flash" or short term, continuous annealing of rod of various carbon and alloy contents in open ring form on a moving conveyor is disclosed in U.S. Pat. No. 3,711,338. The latter process provides some increase in ductility through the start of spheroidization in the various ranges of steel grades disclosed, but it has not gone into general practice because it does not eliminate the need for annealing.
It is also desireable to provide additional treatments of rod other than simple annealing in order to obtain other properties more like those of tempered martensite, but hitherto no process has achieved such an objective.
It is, therefore, an object of this invention to provide a process and apparatus by which steel rod can be rolled and treated and rendered suitable for the specific cold working task intended without conventional heat treatment, as a substitute for a wide range of conventionally heat treated steel rod products. Another object is to provide a means for treating steel rod in direct sequence with rolling whereby physical properties equal to annealed rod, and approaching tempered martensite can be obtained.
Further objects include the provision of methods and apparatus for conserving the expenditure of heat during said processing, for controlling the "structural grain size" by heat treating, and controlling the surface conditioning thereof by cooling and/or coating the rod in sequence with rolling and cooling, whereby physical properties functionally equal to those of conventionally heat treated and or coated products can be obtained in substantially less time and with substantially less expenditure of energy than in conventional heat treating.
Still another object is to provide steel products which, although different in microstructure from conventionally heat treated products of the same composition, are capable of being substituted therefore, or even surpassing same, in the end use.